System and method for modulation of non-data bearing carriers in a multi-carrier modulation system

ABSTRACT

For one aspect of the invention, a method is described for mitigating power spectral density irregularities in a multi-carrier modulation environment. The method involves identifying at least one carrier of a plurality of carriers that is in a non-data bearing state. Thereafter, that carrier is modulated with random data.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/883,554,filed Jun. 16, 2001.

FIELD

The invention relates to the field of communications. In particular, oneembodiment of the invention relates to a system and method formitigating power spectral density irregularities through modulation ofrandom data onto non-data bearing carriers.

GENERAL BACKGROUND

For many years, a number of modulation techniques have been used totransfer data from a source to a destination. One type of modulationtechnique is referred to as multi-carrier modulation (MCM). Inaccordance with MCM, data is split into several data components and eachof these data components is transmitted over separate carriers so thateach individual carrier has a narrower bandwidth than the compositesignal. In general, a “carrier” is an electromagnetic pulse or wavetransmitted at a steady base frequency of alternation on whichinformation can be imposed. Of course, when used in connection withfiber optic medium, the carrier may be a light beam on which informationcan be imposed.

Currently, there exist a number of multi-carrier modulation schemes suchas Orthogonal Frequency Division Multiplexing (OFDM) for example. OFDMsubdivides the available spectrum into a number of narrow band channels(e.g., 100 channels or more). The carriers for each channel may bespaced much closer together than Frequency Division Multiplexing (FDM)based systems because each carrier is configured to be orthogonal to itsadjacent carriers. This orthogonal relationship may be achieved bysetting each carrier to have an integer number of cycles over a symbolperiod. Thus, the spectrum of each carrier has a null at the centerfrequency of each of the other carriers in the system. This results inno interference between the carriers, allowing then to be spaced asclose as theoretically possible.

In many instances, MCM systems are designed to avoid modulatinginformation onto carriers that are unreliable, placing them in a“non-data bearing” state. The carriers are rendered unreliable when theyare experiencing unfavorable channel characterizations such as fading, ahigh degree of interference and the like. Normally, a carrier isdetermined to be “unreliable” based on channel measurements at thereceiver. Since channel characterizations for each unreliable carriermay vary over time, they are periodically monitored through modulationof constant or alternating data (e.g., logic “0”s or “1”s) onto thesecarriers (i.e., an unreliable carrier is modulated with constant data).Non-data bearing carriers may also be used as pilot tones for channelestimation, timing and carrier recovery.

When using an Inverse Fast Fourier Transform (IFFT) to produce multiplecarriers, constant or alternating data modulated carries result inharmonics with concentrated energy at these non-data bearing carriers,which produce Power Spectral Density (PSD) irregularities or peaks atthese carriers.

For example, as shown in FIG. 1, a power spectrum of a transmit signal(e.g., a HOMEPLUG™ packet) using an OFMD modulation technique isillustrated. As shown, four carriers associated with channels 10, 20, 40and 60 are modulated with constant data (e.g., “11” for DifferentialQuadrature Phase Shift Keying “DQPSK”). This causes PSD peaks 100, 110,120 and 130 at those carriers rising approximately eight decibels (8 dB)above the power spectrum 140.

As a result, in order to comply with strict Federal CommunicationCommission (FCC) power level standards and avoid interference to otherusers of the band, the total power of the transmit signal must bereduced. This reduces signal quality (e.g., signal-to-noise ratio)detected at the receiver which, in turn, reduces coverage of thereceiver, data throughput, and the like.

Thus, it would be advantageous to develop a modulation technique thatmitigates PSD irregularities occurring at non-data bearing carriers.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will becomeapparent from the following detailed description of the presentinvention in which:

FIG. 1 is a power spectrum of a transmit signal (e.g., a HOMEPLUG™packet) using an OFMD modulation technique.

FIG. 2 is an exemplary embodiment of a power spectrum of a compositetransmit signal of FIG. 1 produced by a multi-carrier modulation (MCM)system that modulates random data onto non-data bearing carriers.

FIG. 3 is an exemplary embodiment of a communication network utilizingthe invention.

FIG. 4 is an exemplary embodiment of internal logic of a MCM system.

FIG. 5 is an exemplary embodiment of a general block diagramillustrative of logic within a first MCM system (transmitter) thatmodulates non-data bearing carriers with random data for transmission toa second MCM system (receiver)

FIG. 6 is an exemplary embodiment of operations between a transmitterand a receiver in accordance with non-data bearing carrier randommodulation.

DETAILED DESCRIPTION

Herein, various embodiments of the invention relate to a system andmethod for mitigating power spectral density irregularities throughmodulation of random data onto non-data bearing carriers, namelymodulating each carrier currently in a non-data bearing state withrandom data. Herein, this “non-data bearing” state compromises a levelof operation where a carrier is used for other purposes besides datatransmission such as synchronization, carrier recovery, timing recovery,channel characterization, and may be also in an “unreliable” state whenthat carrier is experiencing unfavorable channel characterizations. Ofcourse, non-data bearing carriers may also be used as pilot tones forchannel estimation, timing and carrier recovery.

The embodiments described herein are not exclusive; rather, they merelyprovide a thorough understanding of the invention. Also, well-knowncircuits are not set forth in detail in order to avoid unnecessarilyobscuring the invention.

In the following description, certain terminology is used to describecertain features of the invention. For example, “logic” includeshardware, firmware, software or any combination thereof that performs adesired function on input data. For example, in one embodiment, logiccomprises a processing unit accessing software contained in memory toperform a non-data bearing carrier random data modulation scheme asdescribed in greater detail in FIGS. 5-6. Examples of a “processingunit” include as a digital signal processor, a microprocessor, amicro-controller, an application specific integrated circuit (ASIC), afield programmable gate array, a state machine, combinatorial logic andthe like.

In addition, a “link” is generally defined as one or more physical orvirtual information-carrying mediums to establish a communicationpathway. Examples of the medium include a physical medium (e.g.,electrical wire, optical fiber, cable, bus traces, etc.) or a wirelessmedium (e.g., air in combination with wireless signaling technology). Inone embodiment, the link may be an Alternating Current (AC) power line,perhaps routing information in accordance with a HOMEPLUG™ standard. Oneversion of the HOMEPLUG™ standard is entitled “Release V0.8 MediumInterface Specification” published on or around May 25, 2001.

In general, a “non-data bearing” carrier may occur in any modulationscheme that produces a carrier or pilot tone that is used for otherpurposes besides data transmission, such as synchronization, carrierrecovery, timing recovery or channel characterization for example.Various applications may include, but are not limited or restricted toOrthogonal Frequency Division Multiplexing (OFDM), Frequency DivisionMultiple Access (FDMA), Spread Spectrum, Frequency Division Multiplexing(FDM) or even wavelet based modulation.

Referring to FIG. 2, an exemplary embodiment of a power spectrum of acomposite transmit signal produced by a multi-carrier modulation (MCM)system that modulates random data onto non-data bearing carriers isshown. The power spectrum 200 is produced for a composite transmitsignal 210 having N carriers, where “N” is a positive integer (N≧1). Asshown previously in FIG. 1, the transmit signal 210 features non-databearing carrier numbers 10, 20, 40 and 60, which are represented bylabels 220, 230, 240 and 250, that are now modulated with random data.This method is referred to herein as “non-data bearing carrier randommodulation.” The “random data” may be either (1) truly random in natureand produced by a random bit generator or (2) pseudo-random in natureand produced by a pseudo-random bit generator.

As shown in FIG. 2, the modulation of non-data bearing carriers withrandom data greatly mitigates the presence of power spectral density(PSD) irregularities at frequencies associated with non-data bearingcarriers 220, 230) and 240 and 250. The reduction of PSD irregularitiesis due to the non-periodic nature of the modulated carrier. Thus, poweris not concentrated at these portions of the power spectrum 200, butrather is continuously distributed.

Referring to FIG. 3, an exemplary embodiment of a communication networkutilizing the invention is shown. The communication network 300comprises a plurality of MCM systems 310 ₁-310 _(M) (M≧1) incommunication with a network transceiver 320 via links 330 _(i)-330_(M). These links 330 ₁-330 _(M) may be wired or wireless links. Inaddition, the network transceiver 320 may be further coupled to a link340 operating as networking lines for an establishment (e.g., residence,apartment building, place of business, etc.) as shown. For instance, thelink 340 may be electrical wiring (e.g., AC power line) which data istransmitted over such wiring in accordance with current or futureHOMEPLUG™ standards. Examples of the “network transceiver” include acomputer (e.g., gateway, server, etc.), a router, a switching device, awireless networking access point (e.g., WLAN access point). Of course,although not shown, one or more of the MCM systems 310 ₁-310 _(M) may beconfigured to communicate with other MCM system(s) acting astransceiver(s).

Each MCM system 310 ₁, . . . , or 310 _(M) is a product that supportsthe non-data bearing carrier random modulation scheme, namely themodulation of reliable carriers with the data to be transmitted and themodulation of the non-data bearing carriers with random data. Themodulated carriers are transmitted over a composite channel to thenetwork transceiver 320 (or another MCM system) acting as a receiver.Examples of certain types of MCM systems include various types of MCMmodems (wired or wireless), a computer with wireless connectivity (e.g.,a gateway or server, hand-held “PDA”, a data terminal, laptop, desktop,etc.), a set-top box, a network appliance, a wireless communicationdevice (e.g., phones, pager, etc.) and the like.

Referring now to FIG. 4, an exemplary embodiment of internal logic of aMCM system is shown. The MCM system 310 _(x) includes a processing unit400, an internal memory 410 and a transceiver 420. Configured as anyreadable storage device such as a magnetic, optical, or semiconductorstorage medium, the internal memory 410 may be either physicallyindependent from the processing unit 400 or integrated within theprocessing unit 400. In one embodiment, the internal memory 410 may beimplemented as any type of non-volatile memory such as flash memory,hard disk, on-chip ROM and the like. Of course, it is contemplated thatthe internal memory 410 may include volatile memory or a combination ofvolatile and non-volatile memory. The internal memory 410 storesmulti-carrier modulation software, which enables the processing unit 400to perform non-data bearing carrier random modulation. Of course, it iscontemplated that the MOM system 310 _(x) does not require memory if itsnon-data bearing carrier random modulation functionality is hard-wired.

The transceiver 420 enables modulated carrier signals to be output overa link and destined for receipt by the network transceiver 320 of FIG. 3or perhaps another MCM system (not shown). The transceiver 420 furtherenables channel characterization data to be received from the networktransceiver 320 (or another MCM system) as well.

Referring to FIG. 5, an exemplary embodiment of a general block diagramillustrative of logic within a first MCM system (transmitter) 500 (e.g.,MCM system 310 _(x)) that modulates non-data bearing carriers withrandom data for transmission to a receiver system 550 (e.g., a secondMCM system, network transceiver 320 of FIG. 3, etc.) is shown. For thisembodiment, the first MCM system 500 comprises a multiplexer unit 510, amulti-carrier modulator 520 and a feedback link 530. A (pseudo) randombit generator 540 may be implemented within the first MCM system 500 asrepresented by dashed lines. The receiver system 550 includes amulti-carrier demodulator 560 and a channel estimator 570. These systems500 and 550 communicate over a channel link 580.

Herein, the feedback link 530 provides data from the channel estimator570 of the receiver system 550 as to which carriers of a transmitsignal, if any, possess channel characterization that would cause themto be deemed to exist in an unreliable state. For example, the channelestimator 570 may operate in accordance with blind channel estimation(no knowledge of the transmitted data is required) or data based channelestimation (knowledge of the transmitted data is required so pseudo-RNGused). Both types of channel estimation may analyze signal-to-noiseratio (SNR), bit error rate (BER) and/or other signal characteristics ofeach carrier. The channel characterization of each carrier may becarried out by observing the general characteristics of the signal suchas SNR (blind) or by observing the quality of the values transferred tothe receiver system (data based) 550 and/or comparing them to valuespreviously stored therein. For example, the transmit signal may betransferred in an encoded format, decoded at the receiver system 550 andre-encoded for comparison with the original received signal.

The multiplexer unit 510 uses the data provided from the channelestimator 570, which is referred to as “carrier map,” to select whichoutput ports 511 ₁, . . . , 511 _(R) provide desired transmission dataor random data. Perhaps, as an option, the number of output portscorresponding to the number of carriers forming the transmit signal asshown (e.g., R=N). For clarity, as represented by dashed lines, outputsfrom ports 511 _(i), 511 _(j) and 511 _(k) (where i≠j≠k) are random databecause these carriers are non-data bearing and even deemed to beunreliable. Thus, the input ports 521 _(i), 521 _(j) and 521 _(k) of themulti-carrier modulator 520 receive random data in lieu of transmissiondata. This causes the i^(th), j^(th) and k^(th) carriers to be modulatedwith the random data and transmitted from output port 522 over channellink 580 as part of the transmit signal to the receiver system 550. Thei^(th), j^(th) and k^(th) carriers are modulated in accordance withOFDM, FDMA, Spread Spectrum, FDM, wavelet based or other types ofmodulation techniques.

Referring now FIG. 6, an exemplary embodiment of the operations betweena transmitter and a receiver in accordance with non-data bearing carrierrandom modulation is shown. Initially, the transmitter (e.g., first MCMsystem) sends a channel information request to the receiver (e.g.,second MCM system, network transceiver, etc.) to characterize allcarriers associated with the channel link (block 600). In response toreceiving the channel information request, the receiver analyzes thereceived signal and characterizes the data placed on each carrier (block610). Such characterization may be through analysis of the SNR, BER, andthe like. Thereafter, the receiver determines which carriers are in anunreliable state and outputs a carrier map over the feedback link to thetransmitter (blocks 620 and 630). The carrier map indicates to thetransmitter which carriers are deemed to be unreliable so that no datais placed on to such carriers. In addition, the carrier map is used bythe transmitter to control placement of random data on to those non-databearing carriers.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art. For example, it may bepossible to implement the invention or some of its features in hardware,firmware, software or a combination thereof.

1-27. (canceled)
 28. A method comprising: identifying a carrier of aplurality of carriers that is in a non-data bearing state, including (i)receiving information as to which carriers of the plurality of carriersare to be in a non-data bearing state, and (ii) selecting the non-databearing carrier based on the information; and modulating the non-databearing carrier with random data.
 29. The method of claim 28, whereinthe non-data bearing carrier is a pilot tone.
 30. The method of claim28, wherein the non-data bearing carrier is used for a function besidesdata transmission including channel characterization.
 31. The method ofclaim 31, wherein the non-data bearing carrier is used forsynchronization.
 32. The method of claim 31, wherein the non-databearing carrier is used for carrier recovery.
 33. The method of claim31, wherein the non-data bearing carrier is used for timing recovery.34. The method of claim 28, wherein prior to modulating the non-databearing carrier, the method further comprises producing the random dataas a pseudo-random bit stream.
 35. The method of claim 34, wherein themodulating of the non-data bearing carrier is performed in accordancewith Orthogonal Frequency Division Multiplexing (OFDM).
 36. A methodcomprising: identifying carriers of a plurality of carriers that are ina non-data bearing state; selecting a first non-data bearing carrier ofthe identified carriers in the non-data bearing state; modulating thefirst non-data bearing carrier with random data in order to reduce powerspectral density irregularities at a frequency associated with the firstnon-data bearing carrier.
 37. The method of claim 36, wherein the firstnon-data bearing carrier is a pilot tone.
 38. The method of claim 36,wherein the first non-data bearing carrier is used for a functionbesides data transmission including channel characterization.
 39. Themethod of claim 36, wherein the first non-data bearing carrier ismodulated with the random data being date produced by a random bitgenerator.
 40. The method of claim 36, wherein the first non-databearing carrier is modulated with the random data being data produced bya pseudo-random bit generator.
 41. The method of claim 36 furthercomprising: selecting a second non-data bearing carrier of theidentified carriers in the non-data bearing state; modulating the secondnon-data bearing carrier with random data in order to reduce powerspectral density irregularities at a frequency associated with thesecond non-data bearing carrier.
 42. The method of claim 41, wherein atleast one of the first and second non-data bearing carriers is used forcarrier recovery.
 43. The method of claim 36, wherein prior tomodulating the first non-data bearing carrier, the method furthercomprises producing the random data as a pseudo-random bit stream. 44.The method of claim 43, wherein the modulating of the first non-databearing carrier is performed in accordance with Orthogonal FrequencyDivision Multiplexing (OFDM).
 45. A method comprising: identifying anon-data bearing carrier from a plurality of carriers that are in anon-data bearing state, the non-date bearing carrier being used forchannel characterization; and modulating the non-data bearing carrierwith random data.
 46. The method of claim 45, wherein the non-data beingcarrier being used for channel characterization in determining a levelof interference experienced by the plurality of carriers.